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Converting an RC 256 to single phase

tjb836

Aluminum
Joined
Feb 18, 2012
Location
coldwater,oh USA
I have aquired a Hobart RC 256 three phase welder and have attempted the Haas-Kamp conversion.This welder has two separate transformers fed by thee phase. After requesting assistance from DaveKamp, I first hooked it up as he suggested with the coils paralleled. The measurements were as follows: OCV over 50 as shown on the welders meter. It read very low amps when trying to weld (could not maintain an arc). More to follow.
 
Diagrams

Below are the diagrams I came up with. The first one is the hookup chart inside the welder. The second is a diagram I got from the manual,some of which is not too clear. I will try to get a better one. Number 3 is the first way I connected it and the fourth is what I intend to try next.
 

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Update

Ok, I hooked the welder up as shown in the last drawing and here are the results :
OCV 27 volts
The best I could see the amps were approaching 200 with wire speed at max.
I am working alone so it is difficult to weld and also watch the meters. It is still sputtering some but am not sure if it is a wire feed problem or maybe not enough gas as the torch lead looks as if it had been hanging on a hook for a while. It has a bad kink in it. I will probably have to get a replacement. All in all it looks promising so I will continue on with the project. Any and all feedback will be appreciated .
 
Get a better image...

Get a better image of that second diagram... specifically, the upper left quadrant.

My cursory look at the diagram suggests that since it has two separate cores, there's no need for capacitors... there's no three-phase sequence occuring here... what they did, was just set it up so that it was driving off of all three legs, with just two windings. It's really just pulling from A to B, and B to C.

With the coils wired as shown in #3, it SHOULD be working. Now, the lack of stability makes me suspect that there's a problem with some component somewhere... either the output controls, or a rectifier, or a power contactor... a clearer look at that diagram will provide good direction towards troubleshooting.

Always keep in mind, that when converting a 3-phase welder to single phase, that there's a REASON why the machine ended up in your hands. I just completed a conversion and improved step-by-step writeup of the CP-200, and in my unit, I found that it had been pulled off the production line and put in a storage bin simply because one contact of the welding power contactor had burned. Every time I pulled the trigger, I got a different output voltage. Dismantled the contactor, and the top half of the contact fell out of the works. Slapped in a surplus contactor from my storage warehouse of evil devices, and it works like dream now!
 
Something I've been thinkin' bout here...

Okay, so I've been contemplating this one more, and I wish that you, me, and it was sitting in the shop, so we could team up on it.

This welder uses two separate cores, and they're wired with two primaries together, hooked to the "B" phase, with the far ends of each hooked to A and C respectively. This corresponds to delta, but with one core (A to C) missing.

The outputs are wired just the same... tied in delta to a three-phase rectifier array.


My gut feeling here It will probably work alright wiring A and C of the primaries together, but it'll probably work the rectifiers over pretty good, as they'll be carrying current in a different sequence, so the cyclic currents will likely be high.

If you REALLY wanted to try an synthesize the original working condition, you could try driving one coil directly off the mains, and driving the second coil with some capacitors to phase shift it. Optimally, you'd do it at a 120-degree interval (360 / 3 = 120), but the limit would be 90 degrees.... I'm thinkin' that the end result would be fine at 90, you'd have quadrature going through the rectifiers... which is essentially what happens in an H-K setup.

Off the top of my head, I'm thinking that just installing a pair of caps in series with each lead of one coil would be enough to determine wether improvement is within reach. The mathematics for estimating appropriate capacity will follow the lines of what I did with the CP and SRH's, so get the ballpark, and then give it a try.
 
Ok Dave, you and I must think alike, as I was going to try that very thing. The wife's birthday this week so it may be next week until I get back to it. I will keep all posted.
 
It likes capacitors!

Got a chance to mess around with the welder today. All I had were a couple of 60's left from a previous mod so I threw them in. The OCV came up to 38, which is where it is supposed to be. It also seems to weld better also.
Do you ever measure the voltage across the coils hooked to the caps? When I checked, it measured 260 VAC while at idle, and ~ 90-108 while welding. Just wondering if I should add caps to get the voltage around 240 while welding? Anyway I think were on the right track.
 
Voltage readings...

Yes, I DID take voltage readings, and while it was interesting to look at, I've come to the conclusion that the apparent voltage probably isn't all that indicative of anything, because there are too many variables at play for the reading to be relevant. Current flow IS more relevant, but it's also LOAD dependant... and loading is a very big (and wild) variable... just like an RPC, the amount of phase-shift that occurs, and the voltage and current that appears, is very dependant upon the amount of load.

The best I've found, Is to get them so that the output look-and-feel is close, then work with it a bit, and 'trim' the capacitor banks until the machine and operator are happy.

Your RC is really unique in respect to what I've been doing with the true three-phase machines... and if you've got it working really well, document out all the steps, indicate it on diagrams, and post it!
 
Time for an update

I have been using the welder some over the last week and it works well at higher amps. Where it struggles is at the lower settings. I haven't had time to experiment with more or less capacitance yet,so this will be the next step, when I get some spare time.
Below is a diagram of the primary connections with caps.
 

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Because you have two separate transformer cores, you won't get the same benefit of the H-K conversion- the 'flywheel' of flux circulating through the shared cores. You may find that you'll get better performance with a lesser value at low power, but need more capacity at higher levels. Reason why I suspect this, is because Peter and I based the calculations on the common-core's design FLA-per-winding, and then I experimented backwards to find the 'sweet spot'. See... transformer cores and windings aren't ideal or pure, and in the case of a welding transformer, they've got some design characteristics that wouldn't be employed in a power or signal transformer... when running the transformer at the low end of the power range, the amount of magnetic intensity developed and passed through the core won't be an even proportion to input... and the amount of capacity will obviously skew what's really happening. It's permeability, saturation, reactance, and stuff like that.
 
I had a chance to use the welder this week and it seems to do fine as long as I keep the high-low switch set on high. As far as this goes, I got it for that reason. I have a small wire welder for light guage welding. For the time being I will probably use it the way it is and maybe tinker with it when I get some spare time. Maybe a way to switch caps in and out of the circuit or something like that. I appreciate the help and will add to this thread when I come up with anything to add.
 
...it seems to do fine as long as I keep the high-low switch set on high....Maybe a way to switch caps in and out of the circuit or something like that...

And that may be the slickest solution- if you have the caps split into two banks, use a contactor, powered through the high-low switch... either change the switch to add an extra pole, or find some point in the existing circuit that'd make the switch for you... so it's totally transparent...
 
A better diagram (finally)

I have finally gotten around to finding and posting a better copy of the diagram. Dave, if you see this and have any ideas to make the conversion better I would like to hear them. In using the welder more , I have found most of the problems I was encountering had to do with the liner and rusty wire. Having remedied these problems it runs fairly smooth. I am thinking of trying my original idea of 180 deg. out of phase again to see how it works, now that I found the problem may have been the wire and liner. rc256-12.jpg
 
Depends on the magnetics, really...

I have finally gotten around to finding and posting a better copy of the diagram. Dave, if you see this and have any ideas to make the conversion better I would like to hear them. In using the welder more , I have found most of the problems I was encountering had to do with the liner and rusty wire. Having remedied these problems it runs fairly smooth. I am thinking of trying my original idea of 180 deg. out of phase again to see how it works, now that I found the problem may have been the wire and liner. View attachment 58745

Yeah, having wire/liner and other ancillary problems really screws up the empirical results.

Just so you know... TA and TB appear to have extra windings X3A-X4A and X3B-X4B are 'buck-boost' windings. What happens, is that when the hi-lo switch is thrown in one direction, a certain amount of current from the windings labelled "115vac" are fed to the variable transformer (inverted V shape), then routed to the smaller winding of the 'buck boost' coils. When the switch is thrown to LOW, the polarity 'bucks' the output voltage down. When it's thrown to HI, it 'boosts' the voltage up.

I've been working on a conversion for some Airco CP-series machines that use a similar technique... but somewhat more complex.

The 'catch' of the buck-boost concept, is that there's a phase-relationship that MUST coincide with the buck-boost in order for it to exhibit consistent performance along that variable transformer's range. The variable transformer is PROBABLY a variac, and it may be arranged so that it is, by nature, a 120-degree-interval device. If so, it's somewhat aloof in a 180 degree circuit, but that doesn't mean it won't work. The results of your experimentation should identify how critical that sixty degrees is.

I think the biggest giveaway of a problem, would be detected by an infrared thermometer pointed at various components, and plotting out all the readings over a range of output loads. Substantial temperature rises on one branch of the variac MAY clue us in to what's happening.
 
I know this is an old thread but I just acquired a rc-256 as well and am looking into converting it to single phase. I am definitely no electrician so I'm a little confused about what y'all are saying you changed to get it to work on single phase. If you could help me out I would really appreciate it.
 
Ok here is what I suggest. Go back to post 2 and look at the first picture. That is how the terminal strip for the incoming 230 volt should look like. I would hook one side of the 230 volt incoming power to 102. then hook the other 230 line to both 101 and 103. This can be easily done at the connection block where the power cable connects. Report back your findings.
 
I am having to do the same conversion I have been following this post. My Italian Chinese junker finally quit me. By hooking my machine up like this will I still need caps.
 
If you look back to post 4, Dave did not think it needed capacitors. I never liked the way it ran that way. The reason it didn't run well for me that way turned out to be a bad connection on one of the transformer leads. However I never went back to check operation without caps after I found the bad connection. It worked well with caps so I never pursued it any further. I would try it without caps first.
 








 
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